You should really be concerned by the fact that you just accepted an utterly transparent, zero-effort performance (of her being unable to mount the horse) as reality because that 'reality' reaffirms your preconceptions.

I'm guessing you're self-conscious about your own weight, so you subconsciously tell yourself that skinny is bad/weak every chance you get, as a sort of unhealthy self assurance. But enough speculation--it's not the cause of your bias that's important for you to focus on right now, but the fact of your bias.

Everybody views everything through a filter of their own ideas/experiences/opinions, but it's vital that you keep that filter in check. Your rationality, skepticism, and critical thinking should always be in control, with the "filter" performing a secondary function.

Here, your filter led the way:

"pfft see? see how fuckin weak skinny people are? this is pathetic, she can't even jump on that horse. you're big but you're not fat, you're strong"

and it led you away from the absolutely OBVIOUS and correct conclusion that she is doing a demo and intentionally failing to mount the horse. A bias that is too strong will consistently lead you to the wrong answers for life's questions—a very dangerous & frightful notion.